Aluminum oxide fibers and their production

ABSTRACT

Aluminum oxide fibers of the approximate percent composition by weight: Al2O3 61-98% preferably 76-96% SiO2 1-20% preferably 3-15% P2O5 0-10% preferably 0- 5% B2O3 0-10% preferably 0- 5% MgO 0- 5% preferably 0- 1% C 0.1- 4% preferably 0.5-2.8% The fibers are produced by dry spinning a solution containing a neutral or basic aluminum salt of a monobasic lower carboxylic acid, a hydrolyzed silicic acid ester or organoalkoxysilane and polyethylene oxide having a degree of polymerization of at least about 2,000. The product is then heat treated at a temperature between about 400* and 1,800*C. Phosphorus, boron and magnesium may also be present in the solution and end up in the fiber as oxides.

Mansmann et a1.

[ ALUMINUM OXIDE FIBERS AND THEIR PRODUCTION [75] Inventors: Manfred Mansmann; Ludwig Schmidt, both of Krefeld, Germany [73] Assignee: Bayer Aktiengesellschaft,

Leverkusen, Germany [22] Filed: Dec. 18, 1972 [21] Appl. No.: 316,311

[30] Foreign Application Priority Data Dec. 22, 1971 Germany 2163678 [52] US. Cl 106/65, 106/734, 106/735 [51] Int. Cl C04b 35/10 [58] Field of Search 106/65, 73.4, 73.5; 264/63, 65, 211

[56] References Cited UNITED STATES PATENTS 3,082,099 3/1963 Beasley et al 3,311,481 3/1967 Sterry et al. 3,652,749 3/1972 Sobel et al. 264/63 1451 Feb. 11, 1975 Primary Examiner-.1. Poet Attorney, Agent, or Firm-Burgess, Dinklage & Sprung 5 7 ABSTRACT Aluminum oxide fibers of the approximate percent composition by weight:

A1 0 61-98% preferably 76-96% SiO l-20% preferably 3-1 5% P 0 0-l0% preferably 0- 5% B Q, 0l0% preferably 0- 5% MgO 0- 5% preferably 0 l C 0, l 4% preferably 0.5-2.8%

The fibers are produced by dry spinning a solution containing a neutral or basic aluminum salt of a monobasic lower carboxylic acid, a hydrolyzed silicic acid ester or organoalkoxysilane and polyethylene oxide having a degree of polymerization of at least about 2,000. The product is then heat treated at a temperature between about 400 and 1,800C. Phosphorus, boron and magnesium may also be present in the solution and end up in the fiber as oxides.

11 Claims, No Drawings The present invention relates to novel aluminum oxide fibers and their production.

Since tit-A1 has the highest E-modulus (Youngs modulus) of the aluminum oxide phases, the value being 42,000 kp/mm numerous attempts have been made to manufacture a-Al O in the form of continuous fibers. Hitherto, however, all attempts to achieve good mechanical properties have failed since 0l-Al O is only produced in the course of a heat treatment via several intermediate phases, in the course of which the fibers are greatly weakened. Attempts have already been made, through incorporation of foreign cations, to obtain phases which do not undergo any structural change even at high temperatures, but nevertheless at the same time still contain as high a proportion of A1 0 as possible. According to U.S. Patent 3,271,173 attempts have been made to achieve this aim by incorporating lithium to form Li 0.5 A1 0 a phase which does not undergo any change in modification. While the me chanical properties are distinctly better than those of pure (LY-A1203 fibers, they are still too low for the material to be used as a reinforcing fiber (H. W. Rauch, Sr. W. H. Sutton, L. R. McCreight, Ceramic Fibers and Fibrous'Composite Materials, 1968, page 187).

U.S. Patent 3,503,765 describes a process for the manufacture of polycrystalline fibers based on Al O SiO These fibers mainly consist of mullite (2 SiO .3 A1 0 of crystallite size between 25 and 500, preferably 50 100 A, and contain, as integral constituents, 1 to percent of B 0 P 0 and/or TiO To manufacture them, a solution of metallic A1 in aluminum chloride is mixed with a liquid of low surface tension and subsequently with colloidal SiO or silicic acid and with smaller amounts of boron oxide, phosphorus pentoxide and/or titanium dioxide and the mixture is evaporated to a viscosity suitable for the manufacture of fibers and is subsequently spun. The fibers containing anions are dried and converted into the corresponding oxide fibers by calcination at a high temperature. The achievable E-moduli according to the examples lie in the range of 4,200 to 21,000 kp/mm and are hence substantially lower than the value for the pure (ll-A1 0 :phase.

A disadvantage in the manufacture of the fibers is the low stability of the spinning solutions which, unless special measures are taken, only remain spinnable for a few hours since their viscosity rapidly increases as a result of polymerization. Although polymerization cannot be entirely avoided it can be noticeably reduced by the addition of acetic acid. As the concentration of the solution increases, its stability declines. Hence this method is also less suitable for the manufacture of continuous fibers, since to produce these fibers the spinning solution must be even more concentrated than for the manufacture of short fibers.

Because of their high viscosity, the concentrated solutions can only be filtered with great difficulty. However, filtration before spinning cannot be dispensed with without adversely influencing the strength of the fibers through unavoidably introduced foreign constituents or extremely fine particles of solids formed in the solution itself.

Fibers of A1 0 transition phases, for example the known 'y-A1 O phase, have hitherto not been considered since their existence range is only relatively small (compare, on this matter, J. W. Newsome, H. W. Heiser, A. S. Russelt, H. C. Stumpf, Alumina Properties, Technical Paper No. 10 AlCOA 1960), page 46). Furthermore, these transition phases are only known as micro-porous substances of large surface area since, within their existence range, the temperature does not suffice to obtain dense products through sintering. As has been known for a long time, micro-pores have an extremely adverse effect on the mechanical properties of fibers.

According to the German Offenlegungsschrift No. 2,054,573 y-Al o fibers having very good mechanical properties can be obtained by separating out 10 15 percent of very finely divided carbon in the fibers during the heat treatment of the gel fibers. Through this measure it has proved possible to stabilize the y-Al O phase up to above 1,100C, so to activate the sintering that the fibers show no porosity whatsoever, and to prevent crystallite growth, which would contribute to a weakening of the fibers. Only on heating to temperatures above about 1,200C (it-A1 0 is formed.

A process for the manufacture of aluminum oxide fibers containing a proportion of silicon dioxide and, optionally, small amounts of modifying additives, by spinning a solution of the fiber-forming systems and minor amounts of a dissolved linear polymeric high molecular substance, and subsequent heat treatment, has now been found, which is characterized in that the solution containing a neutral or basic aluminum salt of a monobasic lower carboxylic acid and a hydrolyzed silicic acid ester or a hydrolyzed organoalkoxysilane is spun to fibers in the presence of dissolved polyethylene oxide having a degree of polymerization of above about 2,000, and the fibers are subsequently subjected to a heat treatment up to a maximum of about 1,800C.

The process according to the invention avoids the disadvantages in previously known processes and in particular it is possible thereby to obtain, from relatively dilute and therefore stable, easily filtrable and very simply degassed solutions, continuous fibers which display a high tensile strength and a high E-modulus and are outstandingly suitable for reinforcing plastics, glasses, ceramic materials and metals.

The invention further relates to new, industrially valuable aluminum oxide fibers having good mechanical properties, which are characterized by the following approximate composition by weight:

A1 0 61-98% preferably 7696% SiO 1-20% preferably 345% P 0, and/or B 0 040% preferably 0- 5% MgO 0- 5% preferably 0- 1% C 0.1- 4% preferably 0.5-2.8%

Fibers of the abovementioned composition are obtained by a heat treatment of the fibers obtained in the gel state at temperatures of approximately 400 to 1,800C.

Within the indicated analytical composition, the fibers can, depending on their heat treatment, be characterized by their phase content, which can be determined by X-ray methods; it is possible to differentiate 4 types:

1. Aluminum oxide fibers which are amorphous to X- rays; they are formed by a heat treatment at between about 400C and about 950C. Their density is about 2.8 3.0 g/cm. Typical values of their tensile strength lie between about 80 and 180 kp/mm and typical values of their E-modulus between about 9,000 and 14,000 kp/mm 2. Polycrystalline 'y-Al O fibers; they are produced by a heat treatment at between about 950C and l,l50C. Their SiO- content is not detectable in the X-ray diagram. Their density is about 2.9 3.4 g/cm. Typical values for their tensile strength lie between about 100 and 270 kp/mm and for their E-modulus between about 20,000 and 24,000 kp/mm The crystallite size of the y-Al O phase is about 40 70 A.

3. Polycrystalline 'y-Al O /mullite fibers; they are produced by a heat treatment at between about 1,200 and 1,400C. Their density is between about 3.1 and 3.6 g/cm. Their strength data corresponds to those of the 'y-Al o fibers; their E-moduli are between about 21,000 and 26,000 kplmm The crystallite size of the 'y-Al O is about 40 60 A and that of the mullite phase about 700 1,400 A. fi-Al O is detectable in the X-ray diagram.

4. Polycrystalline y-Al O lmullite fibers; they are produced by briefly rapidly heating to other types to above about 1,400C. They have a density of about 3.4 3.9 g/cm depending on the Si'O content. The tensile strength and E-modulus are between about 80 and 170, and between about 23,000 and 32,000 kp/mm respectively. The crystallite sizes of the y-Al O phase and of the mullite phase are between about 500 and 1,100 A.

In the-abovementioned German Offenlegungsschrift the sintering behavior and the heat stability of the y-Al o fibers are very favorably influenced by the presence of percent of carbon. Surprisingly, it has now been found that it is possible to replace most of the carbon by SiO whereby fibers of greater stiffness and improved oxidation resistance result. The spinning solutions used for this purpose, which in addition to a neutral or basic aluminum formate, aluminum acetate or aluminum propionate also contain a hydrolyzed silicic acid ester or organoalkoxysilane and a small amount of high molecular polyethylene oxide, are stable and of low viscosity, and possess excellent spinnability.

If attempts are made to add the SiO constituent in the form of customary silicic acid or silicic acid sols, stable spinnning solutions are not obtained and/or spontaneous precipitations occur. Such solutions are not suitable for the manufacture of fibers of high tensile strength. Surprisingly, spinning solutions containing, for example,'aluminum oxychloride as the aluminum component, also do not show high values of the tensile strength and E-modulus. This shows that in the manufacture of the aluminum oxide fibers it is necessary, in order to achieve good mechanical properties, to fulfil a series of conditions which cannot be deduced from the previously published data.

The manufacturing process according to the invention requires little technical effort, so that the fibers are inexpensive and apart from being usable as a reinforcing material can also be employed as high temperature insulation, as filters for hot and/or corrosive gases and liquids and as a catalyst or catalyst support.

In detail, the following procedure can be employed to manufacture the fibers according to the invention:

firstly, an aluminum salt solution is produced by dismic acid, at temperatures of between about 40 and C, employing, in general, about 1.7 3.2 moles of the organic acid per 1 mole of Al, so that either basic or neutral salts are present in the solution. lt is, however, also possible, without difficulty, to employ commercially available aluminum formates or aluminum acetates as the starting material.

In cases in which aluminum is used in the form of the metal, it is desirable to form an amalgam with a mercury salt solution since without this the reaction takes place significantly more slowly..The amalgam can be formed, for example, by dipping the metal for a short time into an aqueous solution of HgCland subsequently introducing it into the acid. It is however also possible to add the mercury salt conjointly with the metal to the acid solution. In both cases the mercury employed is recovered in the metallic form after the dissolving process.

The amount of water used during dissolving can be so chosen as to give directly a suitable concentration of A1 0 for the subsequent spinning process. It is however preferred to prepare a substantially more dilute solution, for example with a concentration of about 6 to 9 percent of A1 0 and only to concentrate it to the desired concentration after adding the SiO component. The concentration of A1 0 in the spinning solution is usually between about 8 and 18 percent, preferably between about 11 to 15 percent.

The Si0 constituent of the aluminum oxide fibers according to the invention is present in the spinning solution in the form of a hydrolyzed silicic acid ester or of a hydrolyzed organoalkoxysilane. The hydrolysis of the silicic acid esters, which are'the preferred SiO source, takes place in accordance with the equation p degree of condensation n O to 2 via the intermediate stages of the alkoxypolysiloxanes, up to SiO which remains in colloidal solution. The degree of the hydrolysis, like the degree of crosslinking, depends on the amount of available water. As is shown by the-equation, at least two moles of water are required per mole of silane for the complete hydrolysis of a tetraalkoxysilane. Instead of tetraalkoxysilanes, alkoxypolysiloxanes can also be used.

Methoxy, ethoxy, propoxy and butoxy groups, individually or as mixtures, are used as alkoxy groups. Methoxy and ethoxy compounds are preferred, that is to say, for example, tetramethoxysilane, tetraethoxysilane, hexamethoxydisiloxane, hexaethoxydisiloxane, octamethoxytrisiloxane, octaethoxytrisiloxane and the like, up to polymethoxyand polyethoxy-polysiloxane.

The organoalkoxysilanes are characterized by the general formula wherein R is a lower aliphatic hydrocarbon radical such as, for example, a methyl, ethyl, vinyl or allyl group and OR is the abovementioned alkoxy groupings. Methyl and methoxy and ethoxy groups are preferred, that is to say, for example, the compounds methyltrimethoxysilane or methyltriethoxysilane.

The hydrolysis of these compounds is generally accelerated catalytically by strong acids. Hydrochloric acid in extremely low concentrations, for example about 0.1% by weight or less, relative to the total solution, is preferably added. Usually, a lower saturated aliphatic alcohol, preferably methanol and ethanol, is used as the solubilizing agent for the hydrolysis reaction. For economic reasons, its concentration is kept low. The proportions by weight in the solution to be hydrolyzed are approximately between 5 and 20 percent, preferably 7.5 percent.

The hydrolysis of the silicic acid ester or of the organoalkoxysilane can be carried out directly in the aluminum salt solution. In this case, as a rule, addition of catalyst can be dispensed with. After bringing the components together, the reaction takes place, with stirring and gentle warming, within a few minutes, after which the mixture is stirred for a further /2 to 2 hours. The solutions obtained in this way are waterclear and not opalescent. The silicic acid ester can, however, also be hydrolyzed separately and subsequently mixed, while stirring, with the aluminum carboxylate solution. percent Small additions of SiO already suffice to achieve good mechanical and heat stability of the aluminum oxide fibers. Though in principle high SiO contents are not detrimental to the process of the present invention, they will generally not be used since the filaments then continuously assume the properties of SiO fibres to an increasing extent as the SiO content rises, and in particular the E-modulus approaches the value of SiO of 7,000 kp/mm which is very low in comparison to Al- O For this reason, the amounts of SiO added are as a rule between about I and percet relative to the total oxide sum, and preferably between about 7 and 15 percent.

Before manufacturing the actual spinning solution, yet further additives, with which the properties of the resulting fibers can be modified, can optionally be added to the conjoint dilute solution of the aluminum salt and the silane hydrolysis product. These additives serve to modify the fiber surface with regard to the use of the fibers as a reinforcing material, through the incorporation of basic, neutral or acid constituents, in such a way that depending on the matrix material used a firm bond between the matrix and fiber is formed in each case, which is of great importance for the strength of the composite material. It is furthermore possible, through incorporation of the additives, to give the fiber a surface character which resembles that of glass fibers, so that the adhesion promoters developed for the latter can also be used directly for the aluminum oxide fibers according to the invention. Desirably the additives should not adversely influence the strength of the fibers. Proportions of up to about 5 percent of MgO, preferably 0 to about 1 percent, and of up to about 10 percent of P 0 and/or B 0 preferably 0 to about 5 percent, relative to the total oxide sum, have already proved favorable. These components are usually added in the form of the oxides or of compounds which upon thermal decomposition form the oxides, with volatilization of the remaining constituents. Thus, for example, besides MgO basic magnesium carbonate is also very suitable; P 0 is best used in the form of H 1 0, or as (NI-I HPO and B 0 apart from being used as the oxide, may be added as H BO It has been found that phosphate ions can only be added to concentrated solutions 10% of A1 0 since in more dilute solutions they lead to the precipitation of insoluble aluminum phosphate.

In the previously known processes for the manufacture of inorganic fibers, the salt solutions are concentrated, before spinning, to very high oxide concentrations in order to permit the drawing of fibers through increasing the viscosity. The viscosities necessary for this purpose are however generally so high that the solutions can then only still be filtered with great difficulty and have to be spun using high pressures.

In the process according to the invention, on the other hand, this disadvantage does not apply since the solutions, the oxide concentrations of which are generally between about 8 and 18 percent, preferably about 1 l and 15 percent, are made spinnable by adding small amounts of polyethylene oxide, with the resulting viscosities only amounting to a few poise. The higher the degree of polymerization of the polyethylene oxide the lower is the amount required to obtain spinnable solutions. Degrees of polymerization of above about 2,000, especially above about 5,000, are preferred. The polyethylene oxide content is less than about 2 percent, relative to the spinning solution. At degrees of polymerization of 100,000 and above, additions of less than about 08%, down to about 0.1 percent, already suffice to give solutions of excellent spinnability.

The addition of the polyethylene oxide to the solution containing the fiber-forming elements can be effected, for example, by adding an appropriate amount of solid polyethylene oxide or a concentrated aqueous solution of polyethylene oxide while stirring and homogenizing the spinning solution thus produced by further moderate stirring for several hours. However, it is also possible to add the polyethylene oxide as a solution in warm methanol or a suspension in cold methanol and to produce a clear solution by slow stirring. If desired, the solution can subsequently be degassed in vacuo. After a final filtration, the solutions thus manufactured possess excellent spinnability.

Possible spinning processes are wet spinning and dry spinning processes. In the wet spinning process, the spinning solutions are spun into a suitable precipitation bath, where the sol-gel transformation occurs. The dry spinning process can be carried out, for example, in a centrifugal spinning machine, whereby staple fibers are obtained. However, a conventional dry spinning process is preferably used for the manufacture of continuous fibers. In this, the solutions are spun at room temperature or slightly elevated temperature from a spinning head provided with a plurality of nozzles. The filaments pass through a heated spinning column through which air or nitrogen flows in the same direction as the filaments travel. As a result of removal of solvent, the initially still liquid sol filament changes into a solid gel filament which can easily be handled. At the end of the spinning column the continuous filaments are wound up on a high speed drum. In this way, fiber diameters of between about I and 50 u, preferably between 6 and 15 u, are obtained. In general, the cross-sectional areas are of irregular shape.

Analytically, the glossy white gel filaments thus manufactured generally contain between 35 and 45 percent of oxide. The difference is attributable to the different degree of drying resulting from different column temperatures and different filament gauges. All constituents present in the spinning solution are still present in the gel filaments.

The filaments show no tendency to stick together and possess a strength which easily suffices for their further processing. This further processing consists of a heat treatment which aims at expelling all volatile constituents from the filament and sintering the remaining oxide components together to give a filament of high strength and high E-modulus. For this purpose, the continuous filaments are drawn continuously through a furnace having an appropriately designed temperature profile. i

To manufacture staple fibers, the oxide fibers can subsequently be passed to a mechanical cutting device, where they are cut to the desired fiber length. It is however, also possible to cut the gel fibers before the heat treatment (in which case the shrinkage of the fibers during the heat treatment must be taken into account with regard to the final length) and either to feed them continuously, for example on grids or belts, through a tunnel furnace,"or to heat them discontinuously-to the desired final temperature, for example in a chamberlytic decomposition of the organic constituents, which is largely complete at about 400C. In thecourse ofthis decomposition, the fibers generally shrink to about 60% of their original length. The fibers are amorphous to X-rays up to high temperatures. As a result of the presence of SiO the formation of y-Al O which otherwise takes place at about 500C, is greatly retarded;

the y-phase only forms bet-ween about 950 and 980C in the form of extremely fine crystallites the size of which is generally about 40-70 A. The SiO componentat the same time remains amorphous and cannot be detected by X-rays. As the temperature rises further, the formation of mullite is observed to an increasing degree in the range from about l,l to l,300C; the higher the SiO content of the filament, the lower is the temperature at which this reaction starts. Finally, the conversion of 'yinto a- Al O (corrundum) occurs. This transition is also displaced towards higher temperatures by the presence of SiO While it takes place at between about 1,000 and l,l00C in pure A1 0 it is raised to above about 1,350C at SiO contents. of, for example,

rapid heating of y-Al O fibers to temperatures above the transition point, to obtain corundum fibers having good strengths and extraordinarily high E-moduli. Re action times of about 5 seconds to 5 minutes at temperatures of between about 1,400 and 1,800C have proved particularly suitable. Apart from being determined by their composition, the mechanical properties of the fibers are decisively determined by the nature of the heat treatment. High strength values and E-moduli values are obtained if the-gel filaments are heated to the desired final temperature'at a constant heating rate of between about 5 and C per minute. Lower rates of heating can be used but they are less favorable for high yields. The best results are achieved if the temperatures rise is matchedto the course of the decomposition of the fibers in such a way that the temperature rises more slowly in the regions where much gas is split off than in the regions in which no reaction takes place. Thus it is not possible to exceed about 20C per minute in the region of evaporation of the alcohol and water (up to 150C), and even about 10C per minute during the pyrolysis stage (250 400C), without noticeably damaging the filament structure. On the other hand, when the evolution of gas has ceased it is possible to heat at a substantially higher rate. preferred heating rates are about 2 to 20C/min. up to 250C, about 0.5 to 10C/min. at between 250 and 400C and about 5 to 200C/min. above 400C.

The heat treatment can be carried out in various gas atmospheres with the gas appropriately being passed as a stream through the bundle of fibers, thereby carrying away the substances which have been split off. After completion of the pyrolytic decomposition, the speed of flow can be greatly reduced or a static atmosphere can be used. It was found that a treatment in air up to temperatures of about l,l00C does not give carbonfree filaments. Only above this temperature does the residual carbon content originating from the organic constituents of the gel filament (carboxylate anion, alcohol and polyethylene oxide) burn away, this residual content amounting generally to between about 0.1 and duction in the fiber strength. On the other hand, filaments which have only been heated in air to temperatures at which the carbon remains in the filament possess good mechanical properties. Accordingly, aluminum oxide fibers which are amorphous to X-rays and contain C, and polycrystalline -y-Al O fibers which contain C, are producible by heat treatment in air, since temperatures below l,l00C suffice for their manufacture.

For the manufacture of polycrystalline y-Al O /mullite or a-Al O /mu1lite fibers it is necessary to use higher temperatures. In order to avoid the carbon being burnt away at these temperatures, the treatment is carried out under an inert gas, preferably nitrogen. For

economic reasons it is desirable to allow the first part of the heating to take place in air and'only to carry out the treatment above about l,l00C under an inert atmosphere. It is however also possible to displace the onset of the burning away of carbon towards higher temperatures by reducing the partial pressure of oxygen by rnixing inert gas and air. In the case of the filaments obtained under inert gas again the carbon is burnt away by air or oxygen only above about l,l00C. The fibers according to the invention are completely stable to oxidation below about 900C, even on prolonged heating.

The cross-sections of the fibers are generally irregular in shape so that it appears more reasonable to state the average cross-sectional area rather than the average diameter. The cross-sectional areas can vary within wide limits depending on the predetermined spinning conditions. Typical values lie between about 30 and 180 1. The irregular shape of the fiber cross-sections makes the fibers according to the invention very particularly suitable for reinforcing purposes since they considerably improve the adhesion between the fiber and the matrix in comparison to the known reinforcing materials, which are mostly circular. This expresses itself EXAMPLE 1 27 g of aluminum strip (99.99 percent) which has beforehand been amalgamated by dipping into 5 percent by weight HgCl solution were dissolved at 40C in a solution of 141 g of 98 percent strength formic acid in 800 g of water, while stirring and using reflux cooling. After completion of the solution process, which required about 5 hours, the solution was freed of-the mercury formed by filtration and 4.9 g of a 20.4 percent strength silicic acid sol were added.

A stock solution of this silicic acid sol had been man cross-section of 62 1.1. 173 and 22,500 kp/mm were determined as the average values for the tensile strength and the modulus of elasticity, respectively.

An attempt to convert these 'y-Al O filaments into corundum by annealing at 1,500C in a stream of N for two minutes gave very stiff fibers the carbon content of which, namely 1.5 percent, was practically unchanged. The crystallites had increased in size to 800 A during the transformation into til-A1 0 EXAMPLE 2 An aluminum formate solution manufactured as in Example 1 was mixed with 12.9 g of a 20.8 percent ufactured beforehand by hydrolysis of silicic acid tetraethyl ester by combining 200 g of silicic acid ester, 38 g of methanol, 0.4 ml of N HCl and 48 g of water in an open beaker, while stirring. After stirring for a further half hour, the SiO sol was ready to use.

The combined aluminum formate-silicic acidsolution was concentrated t 50C and a pressure of 14 to 20 mmHg to givea clear concentrate having a total oxide content of 12.75 percent (12.5 percent of A1 0 and 0.25 percent of SiO On stirring in 0.9 percent (relative to the weight of the solution) of polyethylene oxide having a degree of polymerization of about 100,000,

which was dispersed in a little cold methanol, a solution was obtained which after stirring slowly for 15 hours and subsequently filtering could be spun continuously.

Its viscosity at 25C was 270 c? at a shear stress of 19 dyn/cm' Spinning was carried out by spinning the solution as a continuous filament from a reservoir, in which a slight excess pressure of 0.03 atmosphere gauge was maintained, at 25C through a 20 mm long and 0.35 mm wide nozzle into a 3 m high and 7 cm wide column. This filament was dried by a stream of air pre-heated to 70C and flowing in the same direction at the rate of 6 m lhour. The column temperature was maintainedby external heating coils and was 60C at the outlet.

The gel filament issuing from the spinning column was wound up on a totating drum ata speed of 160m/min. The drum was locatedin a cabinet through which the issuing drying air flowed. The atmospheric humidity measured in the vicinity of thedrum was throughout less than 35 percent. The white, glossy filaments obtained in this way,

which do not adhere to one another, were subsequently heated to 1,000C in a stream of N at 6 per minute and then cooled inthe slowly cooling oven. Supple and strong filaments were obtained, which analyticallycon strength SiO sol, manufactured as in Example 1, and the mixture was concentrated to a concentration of 10.3 percent of A1 0 and 0.54% of SiO;. After adding 0.8 percent of polyethylene oxide, as indicated in Ex ample 1, and stirring for two hours, this solution showed very good spinnability.

A part of the gel fibers obtained in accordance with the dry spinning process described in Example 1 was first heated in a stream of N to 1,000C at 6 per minute. After cooling the fibers, which were now black, were introduced under nitrogen into a furnace heated to 1,500C, left there for two minutes and rapidly cooled by removing them from the hot zone. According to the X-ray powder diagram, this treatment resulted in the reaction of A1 0 and Slo to give mullite (crystallite size: 1,100 A) and in the conversion of the remaining aluminum oxide into corundum (crystallite size: 850 A). From the analytical data (94.5 percent of A1- 0 5.0% of SiO and 0.5 percent of C), the composition of the fibers is calculated to be 81.9 percent of corundum, 17.6 percent of mullite and 0.5 percent of C. The filaments at a cross-section of 5911. had a tensile strength of 175 kp/mm and an E-modulus of 30,000 kp/mm".

In a further experiment, the gel fibers were heatedin a stream of N to 1,500C at 6 per minute. This yielded brittle fibers which easily disintegrated and which also consisted of (IL-A1203 and mullite. The crystallite sizes, being 2,000 A and 1,800 A respectively, had however greatly increased relative to the fibers which were only briefly heated to 1,500C.

I EXAMPLE 3 An aluminum formate solution manufactured by dissolving 40.5 g of amalgamated aluminum (99.99 percent) in 141 g of 98 percent strengthformic acid and 800 g of water was mixed with 40.9 g of a 20.8 percent strength Si0 sol manufactured as in Example 1 and the mixture was concentrated to a total oxide concentration of 12.6 percent (1 1.34 percent of A1 0,, and 1.26 percent of SiO After adding 0.66 percent of polyethylene oxide as in Example 1 and stirring for 2 hours, the solution was spun analogously to Example 1 to give continuous filaments.

The gel filaments were heated in a stream of N to 1,100C at a heating rate of 6 per minute and were subsequently allowed to cool in the furnace. Black, glossy, very supple and strong filaments were thus obtained, which contained, as the sole crystalline phase, 'y-Al O with an average crystallite size of A. The SiO constituent was not detectable in the X-ray powder diagram. Analytically, the composition was found -to be 88.1 percent of A1 0 9.8 percent of SiO and 2:1

sectional area, had an average tensile strength of 222 kp/mm and an E-modulus of 20.500 kp/mm Their density was found to be 3.0 g/cm.

EXAMPLE 4 up to 250C, at 3 per minute between 250 and 400C and at 22C per minute above 400C up to 1,350C. After reaching the final temperature, the filaments were removed from the hot zone and in this way rapidly cooled to room temperature.

After the treatment in the stream of N black and very supple filaments resulted, which were very easy to handle. Analytically, the filament composition was found to be 87.9 percent of A1 0 9.7 percent of Si0 and 2.4 percent of C. The X-ray diffractogram showed, surprisingly, that the conversion to the corundum phase had not yet taken place. The greatly broadened reflections indicated the presence of 8- and/or considerably disordered -Ano, in addition to a little mullite. The mechanical properties of the 30 p? cross-sectional area fibers were excellent, with average values of the tensile-strength of 275 kp/mm and of the E-modulus the fibers only possessed moderate mechanical proper,-

ties; they were brittle and very fragile.

If, on the other hand, the heat treatment in air was already brokenoff at 950C, though in other respects the heating program remained the same, and the filaments were annealed for a further hour at this temperature and subsequently slowly cooled in the furnace as it cooled, burning away of thecarbon-couldlargely be avoided. The black, supple fibers thus obtained contained 88.8 percent of 'y-Al O having an average crys tallite size of 65 A, 9.9 percent of SiO amorphous to X-rays, and 1.3 percent of carbon. The average values of the tensile strength and of the'E-modulus were. l70 kp/mm and 20,000 kplmm 'respectively.

In a further experiment, the same stepwise heat treatment of the gel filaments in air was already broken .off

' at 900C, after which the filaments slowly cooled in the furnace. In this way, filaments amorphous to X-rays were obtained, the analytical composition of which was unchanged compared to the filaments obtained at 950C. The valueof the average tensile strength,bein'g 170 kp/mm, was also unchanged, but the E-modulus of 13,000 kp/mm was distinctly lower.

EXAMPLE 5 concentrated at 50C under a pressure of 14 to 20 mm Hg up to a concentration of l 3.95,percent of A1 0 and 1.55 percent of SiO AFter adding 0.70 percent of polyethylene oxide, as indicated in Example 1, and stirring for two hours, the solution was spun in accordance with the dry spinning process described in Example 1.

The white, glossy gel fibers were subjected to the stepwise heat treatment indicated in Example 4, in a stream of N In this treatment the temperature was raised to 1,100C after which the filaments slowly cooled in the furnace while it cooled. The black, glossy filaments thus obtained consisted of 89.2% of 'y-Al O (crystallite size 60 A), 8.9 percent of SiO which was amorphous to X-rays and 1.9 percent of carbon. At fiber average fiver cross-section of 6211. the tensile strength was 205 kp/mm and the E-modulus was 22,000 kplmm EXAMPLE 6 ,To a solution of basic aluminum formate, obtained as in Example 3, 42.7 g of a silicic acid sol with 20,2 percent by weight of SiO manufactured in accordance with Example 1 were added. After concentrating the solution to a total oxide content of 14.4 percent (12.9 percent of A1 0 and 1.5 percent of SiO 1.53 g of 86.7 percent strength phosphoric acid were added together with sufficient water to give an oxide concentration of 13.5 percent (12.0% of A1 0 1.35% of SiO and 0.15 percent of P 0 0.7 percent of polyethylene oxide having a degree of polymerization of about 100,000 was stirred into this solution as described in Example-l, and was homogeneously distributed by stirring for a further two hours. After a last filtration, the solution was continuously spun as in Example 1. The heat treatment of the gel filaments was carried out in a stream of nitrogen, at a heating rate of 6C per minute, up to a final temperature of 1,100C, after which thematerial cooled slowly, with the furnace switched off. a

The resulting black, supple fibers consisted of 87.1 percent of A1 0 in the 'y-form (crystallite size: A), 9.8 percent of SiO; amorphous to X-rays, 1.1 percent of P 0 and 2.0 percent of carbon. At a filament crosssection of 4211. the filaments attained an average tensile strength of 234 kp/mm and a E-modulus of 23,000 kplmm EXAMPLE 7 44.6 percent of a silicic acid sol with 20,2 percent Si0 manufactured according to Example 1 and 4.78 g of boric acid (H were successively added to a solution of basic aluminum formate, containing 76.5 g of A1 0 manufactured as in Example 3, after which the mixture was concentrated to a total oxide concentration of 13.0 percent (11.3% of A1 0 1.3 percent of SiO and 0.4 percent of B 0 After adding 0.7 percent of polyethylene oxide, corresponding to Example 1, stirring for three hours and subsequently filtering, this solution could be spun as described in Example 1.

The heat treatment of the gel filaments consisted of continuous heating to 1,000C in a stream of nitrogen at a heating rate of 6C per minute, followed by 2 minutes annealing at l,500 C under nitrogen. The analysis and examinations by X-ray diffraction gave the following composition of the black, glossy fibers: 84.7 percent of A1 0 10.0 of SiO,, 3.0 percent of B 0 and 2.3 percent of C. Accordingly, the fibers consisted of 59.3

13 percent of corundum (crystallite size: 855 A), 35.4 percent of mullite (crystallite size 535 A), 3.0% of 13 and 2.3 percent of carbon. The tensile strength and E- modulus were 175 and 24,000 kp/mm respectively. The filaments had an average cross-section of 63;.t

EXAMPLE 8 1.08 g of MgCO .H O (39.3 percent of MgO), in the solid form, were first introduced into a solution accord ing to Example 3, which contained 76.5 g of A1 0 in the form of basic aluminum formate, and after this had dissolved 44.4 g of a 20.2 percent strength SiO sol manufactured according to Example 1 were added. The solution was concentrated to a total oxide concentration of 12.6 percent (1 1.2 percent of A1 0 1.3 percent of SiO and 0.1 percent of MgO) and after adding 0.7 percent of polyethylene oxide in accordance with Example 1, stirring for two hours and subsequent fine filtration, the solution was spun.

The gel filaments were first heated in a stream of nitrogen to 1,000C at 6C per minute and after slowly cooling were annealed for 2 minutes at 1,500C, again under nitrogen. Black, matt-glossy fibers containing 86.7 percent of A1 0 10.2 percent of SiO 0.5 percent of MgO and 2.6 percent ofC were obtained, which accordingly consisted of 60.7 percent of corundum (crystallite size 815 A), 36.2 percent of mullite (crystallite size 900 A), 0.5 percent of MgO and 2.6 percent of C were obtained, which accordingly consisted of 60.7 percent of corundum (crystallite size 815 A), 36.2 percent of mullite (crystallite size 900 A), 0.5 of MgO and 2.6 percent of carbon. The average cross-section of the filaments was 26 p and the average values of their mechanical properties were 170 kp/mm for the tensile strength and 22,000 kp/mm for the E-modulus.

EXAMPLE 9 To manufacture fibers of a higher Si0 content, 36.75 g of silicic acid tetraethyl ester were hydrolyzed in an unfiltered solution of 50.6 g of aluminum (99.9 percent) in 1,050 g of H 0, 176.25 g of 98 percent strength formic acid and 141 g of methanol, in accordance with the procedure described in Example 5. After filtration, the mixture was concentrated to a total oxide concentration of 13.1 percent (1 1.1 percent of A1 0 and 2.0% of SiO at 50C and a pressure of about mm Hg, and an easily spinnable solution was obtained by adding 0.7% of polyethylene oxide in accordance with Example 1.

The gel filaments obtained after continuous dry spinning were heated in a stream of nitrogen to 1,000C at a heating rate'of 6C per minute and subsequently left to cool in the furnace. Thereafter, they contained 84.2% of y-Al O with a crystallite size of 60 A, 14.8% of SiO, amorphous to X-rays and 1.0 percent of carbon. The tensile strength and E-modulus were 220 and 23,000 kp/mm respectively; the filament cross-section was 4011..

The y'AI O filaments thus obtained were subsequently subjected to the 2 minutes high temperature treatment in a stream of nitrogen at 1,500C which has already been described several times. In the course thereof, the SiO, reacted to give mullite (52.7 percent) and the remaining A1 0 was transformed into corundum (46.7 percent). The crystallite sizes of the two fiber components were determined to be 1,000 A (mullite) and 700 A (corundum). The average tensile strength had declined to kp/mm after this treatment, while the formation of mullite and corundum caused a rise in the E-modulus to 27,000 kp/mm EXAMPLE 10 According to the X-ray diffractogram the fialments predominantly consisted of mullite in addition to a smaller amount of 'y-Al O The phase distribution can be calculated from the analytically determined contents (80.8 percent of A1 0 17.1 percent of SiO and 2.1 percent of C) to be 60.5 percent of mullite and 37.4% of y-Al O At a fiber cross-section of 211i. the average tensile strength was kp/mm and the E- modulus was 20,500 kp/mm".

EXAMPLE 1 l Unidirectionally reinforced composites were manufactured red with epoxide resin, using the -y-Al O fi bers obtained according to Example 5. A hot-curing epoxide resin based on epichlorohydrin and bisphenol A (epoxide equivalent weight from to was used, with hexahydrophthalic anhydride as the curing agent and dimethylbenzylamine as the accelerator.

To manufacture the composites, the fibers impregnated with the liquid resin were introduced unidirectionally into rectangular steel molds which had beforehand been sprayed with polyfluoroethylene as the release agent. The excess resin was expelled at the open ends of the mold, in the direction of the fibers, by slowly lowering a ground male mold section, after which the mold was heated for one hour to 120C to cure the resin. The resulting laminates, of size 220 X 6 x 4 mm, were subsequently subjected to a flexing test, with the ratio of span to thickness being 50. At a fiber content of 32 percent by volume the flexural strength of the laminate was found to be 56 kp/mm and the E- modulus 7,200 kplmm The strength of the pure matrix material (13.5 kp/mm was accordingly more than quadrupled while the stiffness (280 kplmm was increased approximately 26-fold. The determination of the block shear strength in the direction of the fiber axis gave 7.8 kplmm which led to the conclusion that there was a good bond between the fiber and the resin even without the use of adhesion promoters.

It will be appreciated that the instant specification and examples are set forth by way of illustration and not limitation, and that various modifications and changes may be made without departing from the spirit and scope of the present invention.

What is claimed is: I

l. A process for the production of aluminum oxide fiber which comprises forming a solution containing a neutral or basic aluminum salt of a monobasic lower carboxylic acid, a hydrolyzed silicic acid ester or organoalkoxysilane and about 0.1 to 2 percent by weight of polyethylene oxide having a degree of polymerization of at least about 100,000 said solution having an 4. The process of claim 3, wherein the hydrolysis is effected directly in the solution of the aluminum salt. r

5. The process of claim 1, wherein the solution further contains phosphorus and boron in an amount such that their sum, expressed as P B 0 is about 1 to percent by weight based on total oxides.

6. The process of claim 1, wherein the solution fur- 16 ther contains magnesium in up to about 5 percent by weightexpressed as MgO based on total oxides.

7. The process of claim l, wherein the filamentary structure is heated to a temperature of at least about 800C.

8. The process of claim 7, wherein the heating takes place at a heating rate of about 2 to 20/min. up to about 250C., at a heating rate of about 0.5 to l0/min. between about 250 and 400C., and at a heating rate of about 5 and 200/min. above about 400C.

9. The process of claim 8, wherein the filamentary structure is first heated to about 800 to l,200C. and is subsequently annealed for about 5 seconds to 5 minutes at a temperature above about l,400C.

10. 'The process of claim 7, wherein the heating is carried out under an inert gas.

11. The process of claim 7, wherein heating is carried ,out in an atmosphere containing oxygen up to about '1,l00C. and under an inert gas above about l,l00C. 

1. A PROCESS FOR THE PRODUCTION OF ALUMINUM OXIDE FIBER WHICH COMPRISES FORMING A SOLUTION CONTAINING A NEUTRAL OR BASIC ALUMNINU SALT OF MONOBASIC LOWER CARBOXYLIC ACID, A HYDROLYZED SILICIC ACID ESTER OR ORGANOALKOXYSILANE AND ABOUT 0.1 TO 2 PERCENT BY WEIGHT OF POLYETHYLENE OXIDE HAVING A DEGREE OF POLYMERIZATITON OF AT LEAST ABOUT 100,000 SAID SOLUTION HAVING AN ALUMINUM CONTENT CALCULATED AS AL2O3 OF ABOUT 8 TO 18 PERCENT BY WEIGHT AND SIO2 CONTENT OF ABOUT 1 TO 20 PERCENT BY WEIGHT ON TOTAL OXIDES IN SAID SOLUTION, DRY SPINNING SAID SOLUTION TO FORM A FILAMENTARY STRUCTURE, AND HEAT TREATING SAID STRUCTURE AT A TEMPERATURE UP TO ABOUT 1,800*C.
 2. The process of claim 1, wherein the aluminum salt is aluminum formate.
 3. The process of claim 1, wherein the silicic acid ester or organoalkoxysilane is at least one member selected from the group consisting of tetraethyl silicate, ethoxypolysiloxane, methyltrimethoxysilane and methyltriethoxysilane.
 4. The process of claiM 3, wherein the hydrolysis is effected directly in the solution of the aluminum salt.
 5. The process of claim 1, wherein the solution further contains phosphorus and boron in an amount such that their sum, expressed as P2O5 + B2O3, is about 1 to 10 percent by weight based on total oxides.
 6. The process of claim 1, wherein the solution further contains magnesium in up to about 5 percent by weight expressed as MgO based on total oxides.
 7. The process of claim 1, wherein the filamentary structure is heated to a temperature of at least about 800*C.
 8. The process of claim 7, wherein the heating takes place at a heating rate of about 2* to 20*/min. up to about 250*C., at a heating rate of about 0.5 to 10*/min. between about 250* and 400*C., and at a heating rate of about 5 and 200*/min. above about 400*C.
 9. The process of claim 8, wherein the filamentary structure is first heated to about 800* to 1,200*C. and is subsequently annealed for about 5 seconds to 5 minutes at a temperature above about 1,400*C.
 10. The process of claim 7, wherein the heating is carried out under an inert gas.
 11. The process of claim 7, wherein heating is carried out in an atmosphere containing oxygen up to about 1,100*C. and under an inert gas above about 1,100*C. 